Use CUDNN softmax implementation
Use CUDNN softmax implementation

Summary: The caffe2 implementation of bare Softmax() has a race condition that wipes out the numerical stability trick. Use the CUDNN implementation instead

Reviewed By: urikz

Differential Revision: D4831298

fbshipit-source-id: d11b1de700e3954629e7ed43225a2416c27b3252

bwasti/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,Yangqing/caffe2,Yangqing/caffe2,Yangqing/caffe2,sf-wind/caffe2,davinwang/caffe2,sf-wind/caffe2,bwasti/caffe2,bwasti/caffe2,davinwang/caffe2,bwasti/caffe2,bwasti/caffe2,pietern/caffe2,xzturn/caffe2,xzturn/caffe2,sf-wind/caffe2,pietern/caffe2,caffe2/caffe2,pietern/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,davinwang/caffe2,davinwang/caffe2,xzturn/caffe2,pietern/caffe2